Atmospheric pollutants are those gases, particles, radicals and other molecules that make their way into the atmosphere from other sources or form in the atmosphere from the chemical reactions of other molecules and energy sources. In general, atmospheric pollutants can damage the atmosphere by contributing to the “greenhouse effect,” by breaking down the ozone layer, or by contributing to incidents of asthma and breathing problems. These pollutants are not merely confined to the outside, but can also be found in buildings. For example, many buildings have loading docks near the air intake system. When a truck pulls up to the loading dock, the truck exhaust can be pulled into the air intake system for a building and pollute the indoor air. There are also sources of atmospheric pollutants that originate from materials inside a building, such as carpet, paint, and commonly used chemicals.
Oxides of nitrogen are a group of six compounds. Two members of this group: nitrogen monoxide (NO) and nitrogen dioxide (NO2), together referred to as NOx hereinafter, are reactive species that are important because they are problematic atmospheric pollutants and subject to regulatory control. The gases are regulated because of the large quantities produced through combustion and other chemical reactions and because of their adverse effects in atmospheric chemistry. More than 2 million tons of NOx were generated within the United States in 2011. Combustion typically produces 95% NO and 5% NO2. Nitrogen monoxide, NO, is a significant reactive species in an atmospheric system, along with being present in several types of waste gases. It is the key component in the chain oxidation of organics, which is brought about initially by the radical product of the reaction of hydroxyl radical with organic compounds then adding an ozone molecule to the open radical site. NO scavenges an oxygen atom from the radical organic species to form NO2. In ambient air, there are other important mechanisms by which NO is quickly converted to NO2.2NO+O2→2NO2 k298K=2.0×10−38 cm6 molecule−2s−1 RO2+NO→RO+NO2 k298K=7.6×10−12 cm6 molecule−2s−1 HO2+NO→OH+NO2 k298K=8.3×10−12 cm6 molecule−2s−1 NO+NO3→NO2+O2 k298K=1.8×10−14 cm6 molecule−2s−1 NO+NO3→2NO2 k298K=3.0×10−11 cm6 molecule−2s−1 Alternative NOx Treatment Methodology
Current methods of cleaning air, such as catalytic oxidation, condensation, absorption, and carbon bed adsorption, are in general bulky, expensive, and maintenance intensive. Therefore, a process that could address at least one of these problems found with the currently used methods would be a beneficial next step in the development of better technology for air quality control. An ideal process can control both low and high concentrations of NOx in the air to be treated.
Carbon Bed Adsorption
Carbon bed adsorption, or adsorption by another material, is a process that does not convert the components of waste gases to other compounds as part of the process. Adsorption is an effective way of reducing the concentration of components in a waste gas stream to a low concentration through attachment to a substrate.
The contaminated gas flows through the bed, where the components of the waste gas can be adsorbed onto the bed material. There are, however, several problems with granular bed adsorption. First, the choice of the bed material is one of the critical factors in the success of the component removal. Activated carbon, molecular sieves, activated alumina, and activated silica are common bed materials, although activated carbon is commercially the material of choice. The composition of the bed material influences which waste gas components will be adsorbed and which components will sneak through the system and into the outlet air stream. Therefore, it is helpful if the operator knows the contaminants of the air sample that is being cleaned.
Second, the adsorption technique does not break down the components of the waste gas into smaller and/or other compounds; it only collects them on the bed material. Once the bed becomes saturated, it must be taken off line and cleaned or replaced. The cleaning process can involve simply steam cleaning the bed, called regeneration, or can involve using a solvent combined with steam cleaning to remove captured waste gas components. The waste products from this process must then be collected and disposed of by an environmentally safe procedure. The most common procedure is to separate the waste gas components from the aqueous phase that was produced by the steam cleaning process. This is time consuming, labor intensive and costly.
Another problem with the adsorption technique is that it requires more than one bed in parallel and sometimes in series. The adsorption process requires beds in parallel so that when one bed becomes saturated, it can be taken off line and the other bed is put into subsequent use. Sometimes, it becomes advantageous to put beds in series so that large concentrations of waste gas components can be removed. The operator can also put beds made of different materials in series to target different combinations of waste gases. These adsorption beds are quite bulky, since their average depth is one to three feet, therefore this process can be undesirable if space is limited. The arrangement of beds in series and parallel add to the consumption of time, labor and money in cooling and cleaning of the waste and the bed material.
Absorption
Absorption is the process by which part of a gas mixture is transferred to a liquid based on the preferential solubility of the gas in the liquid. This process is used most often to remove acid stack gases, but it is a complex and costly method of control and often includes the added cost and inconvenience associated with the removal of other innocuous components of waste gases. The high cost of the process is based on the choice of the absorbent and the choice of the stripping agent. Absorption is limited in its utility and not widely implemented in small industrial settings.
Electrical Discharges
Plasmas are electrical discharges that form between electrodes. There are five general classes of non-equilibrium plasmas that can be used in some capacity for chemical processing, including synthesis and decomposition: the glow discharge, the silent discharge, the RF discharge, the microwave discharge, and the corona discharge. Each class is specific based on the mechanism used for its generation, the range of pressure that is applicable during its use, and the electrode geometry.
While electrical discharges are effective in breaking down components of waste gases into other compounds and components, they require power sources (in some cases a significant one), may not be able to handle industrial scale treatment without honeycombed and serial designs of the discharges, and are generally designed to combat complicated waste gas streams that comprise various components, including ozone, NOx and volatile organic compounds.
Wet Scrubbing
For waste gas streams that contain a significant amount of NOx, whether it is an original contaminant or the result of chemical conversion of a volatile organic component, conventional technologies, such as those described earlier, may not be able to efficiently handle the NO load on an industrial scale. Wet scrubbing technologies can handle waste gas streams with significant amounts of NOx. Conventional wet scrubbing technologies for industrial scale NOx treatment typically treat the NOx with two, three or more-stage wet scrubbing systems. The most common currently used is a three-stage process: Stage 1 converts NO into NO2. Stage 2 chemically transforms the NO2 into other nitrogen containing compounds. Stage 3 removes odors created in the second stage. Literature shows a number of chemical reactants, some of which are outlined herein, that are utilized in this and other multi-stage NOx treatment technologies. These include nitric acid and hydrogen peroxide; sodium hydrosulfide and sodium hydroxide, hydrogen peroxide, ozone gas, sodium chlorite solution; and ferric salt solutions and others. All of these have pronounced limitations in operating costs, equipment costs or removal efficiencies.
Catalytic Reactions
Catalysts that can reduce NOx into innocuous nitrogen compounds are effective on NOx waste gas streams with low oxygen concentrations, temperatures between 230° C. and 350° C., devoid of heavy metals that poison the catalyst and sulfur compounds that tend to interfere with the catalysts. However, most industrially produced NOx waste gas streams do not meet these requirements. Thus, the catalyst technology is not applicable.
To this end, it would be desirable to develop a method that converts NOx in a waste gas stream, and the related apparatus and processes thereof, wherein some embodiments of the method, apparatus, and/or process, when compared to certain known technologies, achieves at least one of the following goals: a) can operate on an industrial scale, b) requires less significant amount of energy from outside sources, c) can process waste gases in the gas phase with low, medium and/or high amounts of humidity (including liquid and/or aqueous phase materials), d) can process waste gases using a liquid stream (e.g. an aqueous stream), e) can treat waste gases containing sulfur, sodium and/or other metal containing compounds f) is more cost efficient relative to the scale of the process g) is easier to install and operate, and h) can effectively operate as a single-stage or two-stage unit.